There are many mechanical structural component parts which must absorb high compressive or tensile forces over long periods of time in a pretensioned state, such as cables, bars, wires and screws. A heavy-duty dowel or bolt, for example, is a structural component part of this kind. Knowledge about the pretensioning in such a dowel is important for achieving controlled conditions in fastening. Dowels which are anchored and pretensioned in concrete in a friction-locking manner can relax with respect to this pretensioning as a result of aging and corrosion of the concrete.
At present, an indirect method is most often used for determining the pretensioning, namely measurement via torque. In principle, corrective measures can be taken by monitoring the torque of an expanding screw 1 and by after-expansion of the expanding element 2, as is illustrated by FIG. 1. However, undefined frictional influences (static or sliding friction) prevent a defined adjustment of the pretensioning already when setting the dowel D. But the torque is also not an unequivocal function of pretensioning and can only be measured when tightening, i.e. setting, the dowel D.
An automatic after-expansion and/or a permanent monitoring would also be desirable already when setting the dowel, particularly in safety-related applications. For this purpose a force sensor must be integrated in the structural component part, e.g. the dowel. Piezoelectric systems (quartz force measurement rings) and systems with strain gauges (DSM), e.g. in the form of weighing cells or as annular force transducers are known as such force sensors. However, apart from the high price, such direct measuring instruments for determining pretensioning generally have the disadvantage of an excessively large structural shape for many cases of application. Moreover, the quartz sensor is suitable only for dynamic measurements.
Indirect methods based, for example, on the change in length of the pretensioned threaded rod of the dowel which can be detected e.g. with ultrasonic transit time measurement, are also known. However, this method requires that the dowel be self-calibrating and that its data be recorded.